Nitric Acid-Modified N-Phenyliminodiacetic
Acid – A Total Self-etching Primer for Bonding to Tooth Structure
Dental manufacturers and practicing
dentists need stable, simplified, self-etching primers that will simultaneously
condition both dentin and enamel surfaces and also mediate effective
bonding to these mineralized tissues. Such self-etching primers will
overcome many of the problems of current dental adhesives, promote
conservative dentistry and benefit the general public.
Current dental adhesives systems are characterized by the complexity
of their compositions and bonding procedures. Bonding protocols
for applying a variety of adhesive agents often are technique sensitive,
labor intensive and multi-step in nature. In order to cleanse debris
(smear layer) from the surfaces of prepared teeth and condition
these substrates to accept adhesive agents, many dental adhesive
systems use strongly acidic etchants, e.g., aqueous phosphoric acid.
These aggressive conditioners have a propensity to over-etch mineralized
tissues, especially dentin which is not as highly mineralized as
enamel and, consequently, lacks the superior buffering capacity
of the latter. Many bonding protocols require rinsing and drying
of the conditioned surface. Conditioning dentin with strong acids
removes the smear layer, demineralizes peritubular and intertubular
dentin and creates a micro-porous surface into which adhesive agents
can diffuse. However, these conditioners have the potential of also
damaging subsurface dentin and reducing the strength and/or durability
of the adhesive interface.
Recent studies at NIST have shown that N-phenyliminodiacetic acid
(PIDAA), a unique imino diacid of moderate acidity (pH=2.5) with
excellent chelation potential for calcium, can be used as a dentin
conditioner. Not only does PIDAA remove the smear layer and create
a micro-porous surface without excessive demineralization or the
need for aqueous rinsing, it also has the ability to prime the surface
for effective interfacial polymerization. Thus, PIDAA combines several
functions in one adhesive agent: substrate conditioning for infiltration
and surface activation for polymerization. Several, simplified,
two-step dentin bonding systems based on PIDAA have been developed.
However, there is a need for a self-etching primer that is effective
not only with dentin but also with enamel, that is a “total” self-etching
primer. PIDAA itself is only marginally effective as a self-etching
primer for enamel. In a recently completed study we have shown that
the addition of small quantities of strong acids such as nitric
acid to PIDAA solutions results in effective “total” self-etching
primers. SEM analysis of enamel treated with PIDAA primers showed
that effective surface etching occurred (See Figures 1 and 2).
For assessing shear bond strength to bovine enamel and human dentin,
a two-step bonding protocol was used: 1) aqueous acetone (1:1 by
mass ratio) solutions of PIDAA (6.4 %) and nitric acid (NA) at two
concentrations (2.5 % or 1.25 %) were applied to either bovine enamel
or human dentin surfaces for either 30 s or 60 s, rinsed with distilled
H2O and air dried. 2) Five coats of 20 % photoactivated PMGDM (a
carboxylic acid containing monomer, pyromellitic glycerol dimethacrylate)
in acetone, were then applied and light cured 20 s prior to placement
of a photocurable composite (irradiation time=60 s). All % values
represent mass fractions.
Shear Bond Strength (SBS) and Standard Deviation in MPa to Tooth
Structure
Bovine Enamel
Self-etching Primer
Time (s)
SBS(SD)
PIDAA + 1.25 % NA
60
27.9 (9.6)
PIDAA + 1.25 % NA
30
27.3 (8.5)
PIDAA + 2.5 % NA
60
32.8 (6.1)
PIDAA + 2.5 % NA
30
28.0 (6.5)
Human Dentin
Self-etching Primer
Time (s)
SBS(SD)
PIDAA (Control)
60
24.7 (8.4)
PIDAA (Control)
30
24.6 (6.5)
PIDAA + 1.25 % NA
60
28.1 (7.0)
PIDAA + 1.25 % NA
30
27.3 (11.1)
A two-way ANOVA (á =0.05) indicated that there was no statistically
significant interaction between application time and NA concentration
(p=0.4 for enamel and p=0.74 for dentin).
Simplified dental bonding systems,
equally effective on both dentin and enamel, are feasible with certain
acid-modified PIDAA solutions. We now have developed a self-etching
primer applicable to both dentin and enamel by adding small amounts
of stronger acids such as nitric acid to PIDAA. The resultant bond
strengths of composite to enamel and/or dentin were virtually equivalent.
A joint patent application with the American Dental Association Health
Foundation (ADAHF) has been filed covering this technology. Previous
relevant NIST patents have been licensed by ADAHF and sublicensed
by Caulk/Dentsply.
Fig. 1. Enamel surface
after a 30 s application of 2.5 % HNO3 in 6.4 % PIDAA acetone/H2O
solution. Note: selective demineralization to expose the microstructure
of the enamel prisms.
Fig. 2. Dentin surface
after a 30 s application of 2.5 % HNO3 in 6.4 % PIDAA acetone/H2O
solution. Note: the smear layer is demineralized and the dentinal
tubule orifices are exposed.
For more information on this topic:
Antonucci JM, Bennett PA; Method and Composition for Promoting
Improved Adhesion to Substrates. U.S. Patent 5,498,643, March 12,
1996; U.S. Patent 5,690,840, November 25, 1997; U.S. Patent 5,756,560,
May 26, 1998.
Joseph M. Antonucci, Frederick C. Eichmiller and Gary E. Schumacher
NIST Material
Science & Engineering Laboratory - Polymers Division